Biasing scheme for low supply headroom applications

ABSTRACT

Methods and apparatus for improving the current matching within current mirror circuits in applications such as low voltage integrated circuits. Embodiments of the present invention attempt to maintain the proper current ratio between reference and output supplies by adjusting the reference output of the current mirror. An existing reference voltage on the output side of the mirror can be used or a reference voltage can be created to be used for the voltage regulation of the reference side of the current mirror.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This applications claims the benefit of Provisional ApplicationNo. 60/164,988 filed Nov. 11, 1999.

FIELD OF THE INVENTION

[0002] The invention relates to analog circuit design, and in particularembodiments to low voltage integrated circuits in which currentmirroring is employed.

BACKGROUND OF THE INVENTION

[0003] In analog integrated circuitry there is often a requirement toprovide a precise ratio of currents based on a reference current.Providing such currents is commonly accomplished using current mirrors.

[0004] Modern integrated circuits typically operate with reduced supplyvoltages, in order to conserve energy and to accommodate low voltagedigital circuits. As the components within integrated circuits continueto shrink, circuit breakdown voltages typically decrease and supplyvoltages decrease accordingly. Because of the lower supply voltageswithin modern integrated circuits, power supplies used for currentmirrors and other analog circuitry may be constrained to operate withreduced supply voltages. Accordingly, the voltage available for thefunctioning of current mirrors is decreased and performance may suffer.Because of decreasing supply voltages, circuit parameters may have anincreasing effect on the current provided by current mirrors.Accordingly, there is a need within the art for improved biasingtechniques for use with current mirrors.

SUMMARY OF THE INVENTION

[0005] Embodiments of the present invention attempt to maintain theproper current ratio between a reference current and the output currentof the current mirrors. Embodiments of the current invention attempt tomaintain the proper current ratio between the reference current andoutput current of current mirrors through methods applied to thereference side of the current mirror. This method of compensation usingthe reference side of the current mirror may be more effective thanattempting to increase the current in the output sides of the currentmirror, especially in those cases in which the supply voltage of theoutput current side is low. If most of the supply voltage is droppedacross the load, of the output side of the current mirror, no voltageheadroom may be left to perform current regulation necessary to maintainthe proper ratio between reference and output currents.

[0006] Embodiments of the present invention may include such methods asmatching the voltage across the output device in the reference side ofthe current mirror to the voltage drop in the output device of theoutput side of the current mirror. Embodiments of the present inventionmay also include various measures to insure that the internal impedanceof the reference side is proportional to the impedance of the outputside of the current mirror in such a ratio as to maintain the propercurrent ration between the reference current and the output current.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Referring now to the accompanying drawings in which consistentnumbers refer to similar parts throughout:

[0008]FIG. 1A is a graphical representation of an exemplary environmentin which an embodiment of the invention may operate.

[0009]FIG. 1B is a circuit diagram of a current mirroring systemaccording to the prior art.

[0010]FIG. 2 is a schematic of exemplary prior art multiplying currentmirror.

[0011]FIG. 3 is a schematic diagram according to an embodiment of thecurrent invention.

[0012]FIG. 4 is a schematic diagram of an embodiment of the inventionutilizing a multiplying current mirror.

[0013]FIG. 4A is a block and schematic diagram of a further embodimentof the invention, in which a voltage supply is added to further improvecurrent mirror matching.

[0014]FIG. 5 is a schematic diagram of an implementation of the currentmirror illustrated in FIG. 4.

[0015]FIG. 6 is a schematic diagram of an embodiment of the invention,illustrating an arrangement of the output devices of a current mirror,which provide current to a differential input circuit.

[0016]FIG. 7 is a schematic diagram of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017]FIG. 1A is a graphical representation of an environment in whichan embodiment of the invention may operate. In FIG. 1A integratedcircuit 101 includes current source 103 which draws a reference currentI_(ref). The current I_(ref) is duplicated by a mirror current source107, supplying a current of I_(mirror). The mirror current, I_(mirror),is supplied to a load 105. Such a configuration as illustrated in FIG.1A is commonly used within the analog portions of integrated circuits.The current I_(mirror) may be equal to I_(ref), the reference current,or it may be a multiple of I_(ref).

[0018]FIG. 1B is a circuit diagram of a prior art current mirroringsystem. The circuit in FIG. 1B attempts to replicate reference currentI_(ref) 125 in the output branch of the circuitry 123. That is, it isdesirable to make I_(out) the same value as I_(ref). In order to makeI_(out) equal to I_(ref), the voltage across the drain source junction(Vds) of device 115 should equal the Vds of device 117. Because devices115 and 117 are integrated devices, their characteristics are verysimilar. If the drain voltages of devices 115 and 117 are equal, thecurrents through the devices will be essentially equal because the gatesof the devices are at equal potential, that is, .they are tied together.A problem occurs when the common mode voltage at point 123 of thedifferential input circuit 129 drops. When the voltage at point 123drops, device 127 (the upper device of the cascode pair 127 and 117) maynot remain in saturation. If device 127 comes out of saturation, andgoes into triode mode, drain voltage on device 117 will be lower thanthe drain voltage on device 115. Because the drain voltage on device 117is lower, the current through device 117 will be lower than the currentthrough device 115 and the output current I_(out) will no longer matchthe current (or a multiple of the current) produced by the referencesource I_(ref) 125. Differential input circuit 129 is shown for thepurposes of illustration. In practice, any circuit coupled to the mirrorcurrent source I_(out) will experience a similar problem once thevoltage at the output of that circuit, i.e., the voltage at point 123drops sufficiently. The problem is exacerbated in the case where devices115 and 117 are operated in the degenerative mode, in which resistorsare added between the source and ground of devices 115 and 117.

[0019]FIG. 2 is a schematic of an example of a prior art multiplyingcurrent mirror. FIG. 2 is similar to FIG. 1 except that the currentmirroring devices illustrated actually represent multiple devices. Thatis, for example, the output cascode pair 227 and 217 each represent 20devices in parallel. Device 215 represents two devices in parallel.Because the ratio of the number of devices in the reference currentsource to the number of devices in the output current source is 1 to 10the output current I_(out) through point 223 will be 10 times thereference current produced by I_(ref) 225. The same type headroomproblem can occur whether I_(ref) and I_(out) are equal or multiples.So, for example, if the common mode voltage 223 of the differentialinput circuit 229, drops low enough (for example, if I_(nn) 219 andI_(np) 221 drop low), the 20 devices in parallel, 227, may begin to comeout of saturation and enter triode mode. Once the voltage at 223 dropslow enough so that the 20 devices 227 begin to enter triode mode, thevoltage at the drains of the 20 devices 217 begins to decrease. Once thevoltage at the drains of devices 217 begins to decrease, the drainsource voltage across devices 217 follows. When the drain source voltage(Vds) across devices 217 decreases to the point where it is lower thanthe Vds of devices 215, the current through device 217 will decrease.Accordingly, the current in each device 217 becomes less than thecurrent in each device 215 and the current ratio changes due to thelessening of the output current.

[0020]FIG. 3 is a schematic diagram according to an embodiment of thecurrent invention. In FIG. 3, the circuit 329, which is coupled to theoutput current mirror device 319, is again exemplarily a differentialinput circuit. Those skilled in the art will realize that thedifferential input circuit 329 serves as an example of a common loadcircuit but is not limited to a differential circuit. The presentembodiment of the invention is applicable to any type of circuit beingdriven by a current mirror output device 319.

[0021] In FIG. 3 a resistor R309 is added between the source of device300 and the drain of device 317. Resistor 309 is equal to the impedanceof circuit 329, as determined by the parallel combination of resistors313 and 315.

[0022] In the circuitry in FIG. 3, device 319 cannot compensate for thelow voltage at its drain because the low voltage is a characteristic ofthe circuit load. Therefore, to be effective, load compensation willneed to be accomplished within device 317, in the reference side of thecurrent mirror.

[0023] A voltage is placed on the input of device 300 representing thecommon mode voltage (that is, it represents the average voltage betweeninput 302 and input 304 of the differential input 329) of the circuit329. As the voltage at the drain of device 319 changes, so will thevoltage at the drain of device 317. Because the Vds of device 317 willtrack the Vds of device 319, and because the gates of device 317 and 319are tied together, the reference current will track the output currentI_(out).

[0024]FIG. 4 is a schematic diagram of an embodiment of the inventionutilizing a multiplying current mirror. In FIG. 4., the output currentI_(out) is equal to 10 times the current provided by reference generator407 thereby providing the desired current ratio of 10 to 1. The input401 represents a common mode voltage, that is, the average betweenI_(nn) 402 and I_(np) 404. Since the I_(out) of device 419 represents 20devices in parallel, and reference device 417 represents two devices inparallel, a 10:1 ratio results. 409 represents the parallel combinationof the two resistors 413 and 415. Resistor 409 represents 10 times theresistance of circuit 429 or 20 times each individual resistor 413 or415. The impedance of the reference side is N times the impedance of theoutput side of the current mirror (where N is the ratio of the outputcurrent to the reference current).

[0025]FIG. 4A is a schematic diagram of a further embodiment of theinvention, in which a voltage supply is added to further improve currentmirror matching. In FIG. 4A a voltage source 423 has been added. In FIG.4A, just as in FIG. 3, Vds of the reference output device 417 isadjusted to match Vds of the output mirror device 419. The drain voltageof the reference side device 400, however, is different than the drainvoltage of device 403. Voltage source 423 equalizes the voltage on thedrain of the current mirror 400 with the drain voltage of devices 403and 405. By matching the drain voltage of the reference side device 400with the drain voltage of devices 403 and 405, the voltage between thedrain of the driver device 400 and the output device 417 is brought tobe more in line with the voltage between the output devices 425 and thedrain of device 419.

[0026]FIG. 5 is a schematic diagram of an exemplary implementation of acurrent mirror, similar to that illustrated in FIG. 4. In FIG. 5, thecombination of a current source 523, resistance 525 and second currentsource 527, replaces the voltage supply 423 of FIG. 4A. In FIG. 5, theresistor 525 is calculated such that the current I₅₂₃ times theresistance of 525 equals the voltage supply 423 (as illustrated in FIG.4A). In addition, current supply 527 is set equal to current supply 523.From Kirchoff's current laws the sum of currents into a node must alwaysequal 0. Thus current I₅₂₅ minus current I₅₁₇ minus current I₅₁₉ minuscurrent I₅₂₇ equals 0. Likewise, current I₅₂₃ plus I₅₀₀ minus I₅₂₅ mustequal 0. By setting both equations equal to one another it can bedetermined that I₅₁₇ plus I₅₁₉ must equal I₅₀₀. Because devices 517 and519 are in fact FET-type devices, I₅₁₇ and I₅₁₉ are negligible.Therefore, current I₅₀₀ is also negligible. Thus, the desired voltagedrop across resistor 525 can be achieved by considering only the valueof the current sources 523 and 527 and the resistance value of resistor525.

[0027]FIG. 6 is a schematic diagram of an alternate embodiment of theinvention, illustrating an arrangement of the output devices of acurrent mirror providing current to a differential input circuit. InFIG. 6, individual output devices 629 and 619 replace a single outputdevice such as device 519 in FIG. 5. In FIG. 6, the differential inputcircuit 631 has degenerating resistors 613 and 615 coupled together, notin line with the output current. Such an arrangement can increase theheadroom for the output devices of the current mirror. In such anarrangement, however, there could be a larger contribution to thermalnoise of differential pair 631 by the current source devices.

[0028]FIG. 7 is a schematic diagram of a further embodiment of theinvention. In FIG. 7, the circuit 725 which comprises the load for theoutput side of the current mirror, is replicated in the reference sideof the current mirror. Circuit 725 in the reference side of the currentmirror is designated as 725 _(ref). Input 721, in addition to beingcoupled to the gate of device 703, is also coupled to the gate of device703R. Additionally, the signal 723 which is coupled to the gate of thedevice 705 is also coupled into the gate of device 705R in the circuit725 _(ref). In such a manner the circuits 725 and 725 _(ref) are madeelectrically equivalent. By making circuit 725 _(ref) and circuit 725electrically equivalent, the voltage drop across them will be identical.Additionally, the output devices of the reference side of the currentmirror and the output side of the current mirror can be degenerated.That is, resistors 713 and 715 may be added to the circuit. In such away, the current generated in current source 707 is replicated by I₇₂₇in the output leg of the current mirror.

What is claimed is:
 1. A method of providing proportional currents in acurrent mirror, the method comprising: providing a current mirror havinga reference current side which provides a reference current, and a loadcurrent side which provides a load current; detecting an indicationwithin the load current side of the current mirror that the load currentis decreasing; and decreasing the reference current proportional to adecrease in the load current.
 2. A method as in claim 1 whereindetecting an indication that the load current is decreasing includesdetecting an indicating voltage from the load currentside of the currentmirror.
 3. A method as in claim 2 wherein decreasing the referencecurrent further includes using the indicating voltage to decrease thereference current.
 4. A method as in claim 2 wherein detecting anindicating voltage further includes: detecting at least one inputvoltage coupled into the load current side of the current mirror; andcomputing the indicating voltage based on the at least one inputvoltage.
 5. A method as in claim 2 wherein detecting an indicatingvoltage further includes: detecting at least one common made voltagefrom the load current side of the current mirror.
 6. An apparatus forproducing a load current proportional to a reference current, theapparatus comprising: a reference current generator that produces areference current; a load current generator that produces a load currentproportional to the reference current; a sense circuit for sensing atleast one parameter indicative of the load current and producing acontrol voltage from said at least one parameter; and a circuit thatdecreases the reference current proportional to the control voltage. 7.An apparatus as in claim 6 wherein the parameter sensed is a voltage. 8.An apparatus as in claim 7 wherein the parameter sensed is a voltageproportional to at least one voltage input to a load driven by the loadcurrent generator.
 9. An apparatus as in claim 8 wherein the load drivenis a differential input circuit and wherein the parameter sensed is acommon mode voltage of the differential input circuit.
 10. An apparatusas in claim 8 wherein the parameter sensed is a voltage at a junction ofload current generator and the load driven by the load currentgenerator.
 11. An apparatus as in claim 6 wherein the circuit fordecreasing the current in the reference circuit further includes avoltage source electrically in series with the reference generator. 12.An apparatus as in claim 11 wherein the circuit that decreases thereference current decreases the reference current in inverse proportionto the control voltage.
 13. An apparatus as in claim 6 wherein thereference current generator includes an impedance substantiallyequivalent to the load impedance.
 14. An apparatus as in claim 13wherein the reference current generator impedance includes a circuitwhich is a substantial duplicate of the load circuit.
 15. An apparatusas in claim 14 wherein the circuit which is a substantial duplicate ofthe load circuit accepts the same inputs as the load circuit.
 16. Anapparatus as in claim 13 wherein the reference current generatorimpedance is an equivalent impedance.
 17. A circuit to improve trackingin a current mirror having a reference current side and a load currentside, the reference current side having a reference current sideimpedance and the load current side having a load current sideimpedance, the circuit comprising: a semiconductor device through whichthe load current passes; a semiconductor device through which thereference current passes; and a circuit coupled to the output circuitwhich adjusts the voltage across the reference semiconductor device tomatch the voltage across the load semiconductor device.
 18. A circuit asin claim 17 wherein the reference current side impedance is adjusted tomatch the load current side impedance .